Plasma projection is a thermal treatment process permitting producing a surface coating on an object, a member or the like, said coating being then adapted, as the case may be, to fulfill a role as a deposit for anti-wear, anti- corrosion, anti-friction or thermal and/or electrical barrier purposes.
In a manner known per se, plasma projection is a process for thermal coating consisting in introducing a particular material into a plasmagenic gas jet within which the particles are melted and accelerated before striking the surface of the piece to be coated.
Among the different types of particular materials usable in a plasma projection process, can be mentioned metallic powders constituted by a pure metal or an alloy of several metals, composite powders, for example a powder of tungsten carbide in a matrix of cobalt, or ceramic powders such as refractory oxides, for example oxides of aluminum, zircon, chromium, mixed compounds of the aluminum/titanium mixture type. . .
Conventionally, the gases used for plasma projection are particularly binary, ternary or quaternary mixtures of argon, hydrogen, nitrogen and/or helium whose respective proportions vary considerably as a function particularly of the material projected and/or of the material to be coated.
Thus, EP-A-0 451 051 discloses a plasmagenic gas mixture constituted by a ternary mixture of helium, argon and hydrogen, which contains 30 to 700 helium, 10 to 50% argon and a to 25% hydrogen. Moreover, EP-A-0 639 427 discloses itself gaseous mixtures comprising 4 to 5 constituents, namely mixtures comprising argon, helium, hydrogen, carbon dioxide and/or oxygen.
Furthermore, binary argon/hydrogen and argon/helium mixtures have already been disclosed.
At present, the plasma projection processes use different types of torches which can be divided into two categories, namely high power torches and low power torches.
More particularly, the high power torches operate conventionally at high current intensities, typically of the order of 500 to 600 A and high gas flow rates, for example of the order of 50 to 60 l/mn.
This type of torch is conventionally used particularly for the deposit of refractory materials, for example of the zirconium yttrium type.
Conversely, low power torches operate at lower current intensities, generally from about 300 to 450 A, and at lower gas flow rates, for example of the order of 30 to 40 l/mn.
Until now, low power torches have been little used for the plasma projection of refractory materials, except in certain cases, for example when heat output must be relatively low so as to avoid overheating the support.
However, more and more, it is desired to make the use of low power torches generally useful for all types of plasma projection work of refractory materials, so as to obtain results at least comparable to those obtained with high power torches used at present.
However, it has been noted by the inventors of the present invention that the plasmagenic gaseous mixtures used until now with high power torches are not suitable for low power torches. Thus, tests carried out by means of ternary mixtures of argon/hydrogen/helium according to the prior art have shown too high a power, leading to rapid deterioration of the torch.